The entrainment of fly ash particles from the lower furnace of an industrial boiler to the convection sections of the boiler is an inevitable process. The accumulation of these particles in the fireside heat exchanger surfaces reduces the boiler thermal efficiency, creates a potentially corrosive environment at the boiler tube surfaces and, if the accumulation is not properly controlled, may also lead to costly unscheduled boiler shutdowns due to plugging of the gas passages.
Knowledge of the flue gas temperatures across the boiler heat transfer surfaces is therefore an important piece of information that can be used to evaluate fireside deposit characteristics, to improve boiler cleaning operation through intelligent deposit removal processes, and to optimize boiler operation and combustion processes. Conventional temperature sensors positioned in fixed locations on boiler walls or other internal boiler structures do not monitor flue gas temperatures across the boiler heat transfer surfaces. There is, therefore, a continuing need for effective ways of monitoring the internal temperature of flue gasses across heat transfer surfaces inside of industrial boilers.
Sootblowers are by far the most widely used equipment to remove the fireside deposit accumulations in industrial boilers, such as oil-fired, coal-fired, trash-fired, waste incinerator, as well as boilers used in paper manufacturing, oil refining, steel, and aluminum smelting and other industrial enterprises. A sootblower consists of a lance tube with one or more nozzles. During the deposit removal process, the sootblower lance rotates and extends through a small opening in the boiler wall, while blowing high pressure cleaning fluid (e.g., steam, air or water) directed into the tube banks. After the lance is fully extended, it rotates in the opposite direction as it retracts to its original inactive state.
The sootblower carriage consists of one or two electric motor(s), a gearbox and a packing housing. The electric motor is the main drive that moves the lance tube forward and backward during the cleaning cycle. The motor converts electrical energy into rotation motion, which is then used by the gearbox to rotate and move the lance tube along the gear rack. As the steam enters a sootblower, it is directed to four components in the following order: poppet valve, feed tube, lance tube, and nozzles. The lance tube is the main component that travels within the boiler while supplying the sootblower nozzles with high pressure steam directed by jets toward the boiler tubes. The lance travel includes insertion into and retraction from the boiler. During the cleaning process, the lance extends into the boiler and forms a structure similar to a cantilevered beam. Hence, the lance has to be designed to have sufficient strength to support its own weight in a high temperature environment.
To avoid overheating the lance tube during internal boiler operation, the blowing fluid, which also acts as a cooling medium, needs to be supplied continuously to the lance. The minimum amount of the cleaning media required to prevent the lance from overheating is known as the minimum cooling flow. The minimum cooling flow of a lance tube depends on the material, the length of the lance tube, the steam and flue gas temperatures. Knowledge of the lance tube temperatures as the lance is being exposed to hot flue gas inside the boiler is very important to prevent lance tube overheating and to devise emergency sootblower retraction control strategy. A continuing need therefore exists for effective ways for monitoring the temperature of the lance tube as the lance is exposed to hot flue gas inside the boiler.